1. Field of the Invention
The present invention relates to a process for purifying the impure lead contained in recovered lead fixtures and in scraps and processing wastes, with the melting processes being eliminated which are presently essential for the thermal refining or for the preparation of the suitable anodes for the electrolytic refining, in the event when this refining system is adopted.
2. Description of Related Art
As is known, the electrolytic lead refining is carried out in cells to which massive anodes are charged, which are manufactured by melting impure lead and casting it into suitable molds, and cathodes, constituted by thin sheets of lead or stainless steel on which the refined lead is deposited owing to the effect of the electrical field established between the anode and the cathode.
The electrolyte is generally constituted by an aqueous solution of lead fluorosilicate containing free fluorosilicic acid, and the addition of additives in order to obtain a deposit displaying good characteristics.
The massive anodes of the known type suffer from several drawbacks and limitations of practical character: first of all, the anodes which get exhausted have to be removed at pre-established time intervals, with the production cycle being discontinued.
Furthermore, the so-said "anodic residues" which constitute from 20 to 25% of the initial weight have to be melted once more, and this is a further additional cost.
The anodic sludges often get detached from the anodes, get accumulated on the bottom of the electrolytic cell, and must be periodically removed. Furthermore, the sludges can get dispersed throughout the bath and constitute a polluting agent for the deposit.
Then, it should be observed that the anodes to be refined should display a limited level of impurities (Cu, Sn, Sb, As, Bi), the total amount of which does not normally exceed 2-3%, and normally have to be submitted to a pre-refining process, with consequent slagging of 3-5 parts of lead per each part of impurities to be removed.
The present refining system with massive anodes of impure metal displays the characteristic that the anodic surface is very close to the cathodic one, and hence has a very similar current density, expressed as A/m.sup.2.
It derives from the above that the cathodic current density, and, consequently, substantially, the production capacity of the facility, cannot be increased beyond certain threshold values, in order to prevent that anodes do not become passivated, or cathodic deposits of poor quality are obtained.
The presence of sludges which, when a large amount of impurities are present, adhere to the anode, prevents the use of techniques which may increase the lead diffusion coefficient in the double cathodic layer, such as strong circulation rates or stirring techniques, for fear of detaching the layer of anodic sludges, with seriously negative consequences for the purity of the metal deposited at the cathode.
As electrolysis goes on, the layer of anodic sludges reaches considerable thickness, with the anodic dissolution potential being increased. When this anode dissolution potential reaches the value of impurities dissolution potentials, the impurities are dissolved and are deposited at the cathode.
In order to obviate this drawback, either the current density is reduced, or the anodes are frequently extracted from the cells in order to clean them from the sludges.
Most electrolytic lead refineries presently installed operate with a cathodic density of abound 200 A/m.sup.2. When the level of impurities exceeds the normal level of 2-3%, the current density must be drastically reduced, down to 25% of normal values, with dramatic production drops.
Summing-up, the refining system with massive anodes containing high level of impurities suffers from a large number of electrochemical limitations, requires
melting and thermal pre-refining furnaces, a complex casting system, a complex handling system for the new anodes, the anodic residues and the anodes from which the sludges must be removed during the refining cycle.